A conventional liquid crystal driving circuit for carrying out multi-tone display in a liquid crystal display device is disclosed, for example, in Japanese Unexamined Patent Publication No. 118908/1994 (Tokukaihei 6-118908) (published on Apr. 28, 1994). This liquid crystal driving circuit outputs an analog voltage which has been subjected to .gamma. correction, which shows a kinked characteristic in accordance with the optical characteristic of a liquid crystal material in accordance with tone levels. FIG. 15 shows a circuit structure which realizes multi-tone display by this driving technique. A liquid crystal driving circuit of FIG. 15 has a digital signal input of 6 bits and a single analog signal output, and converts a digital signal for multi-tone display into an analog signal for carrying out multi-tone display on a liquid crystal panel so as to output the analog signal thus converted.
This liquid crystal driving circuit includes a reference voltage input terminal 21, an analog switching element 22, an analog switching element 23, a capacitor alley 24, analog switching elements 25a and 25b, an adjuster capacitor 26, a feedback capacitor 27, and an operational amplifier 28. The reference voltage input terminal 21 is a terminal for inputting a plurality of reference voltages. The analog switching element 22 is turned ON or turned OFF by the upper two bits of the input digital signal, and the analog switching element 23 is turned ON or turned OFF by the lower four bits of the input digital signal. The capacitor alley 24 is weighted 2.sup.0 to 2.sup.3 times a basic capacitance. The analog switching elements 25a and 25b carry out initialization of a digital/analog conversion circuit. The adjuster capacitor 26 has the capacitance 2.sup.0 times the basic capacitance, and the feedback capacitor 27 has the capacitance 2.sup.4 times the basic capacitance. The operational amplifier 28 is a differential amplifier circuit. The analog switching elements 22, 23, 25a and 25b are all composed of MOS transistors.
The following briefly explains the operation of the above liquid crystal driving circuit. The input voltages from the reference voltage input terminal 21 are selected by the analog switching element 22, which is turned ON or turned OFF by the upper two bits of the input digital signal so that only adjacent two voltages are outputted to the analog switching element 23.
The analog switching element 23, upon receiving the reference voltages, sends only one of the reference voltages to the capacitor alley 24 in accordance with the lower four bits of the input digital signal.
The analog switching elements 25a and 25b provided for initialization carry out initialization in such a manner that all SW-A are turned ON and all SW-B are turned OFF during initialization so that the charges in the capacitors are all discharged. Thereafter, in the subsequent digital-analog conversion (hereinafter simply "DA conversion"), SW-A are all turned OFF and SW-B are all turned ON.
The reference voltage selected by the analog switching element 23 is applied to one terminal of each capacitor of the capacitor alley 24, resulting in a potential change. This sets off redistribution of charges among the capacitor alley 24, the adjuster capacitor 26, and the feedback capacitor 27. The capacitance of each capacitor of the capacitor alley 24 is set beforehand to the value 2.sup.0 to 23.sup.3 times the basic capacitance in accordance with the weighting of the input digital signal, and therefore the output voltage Vout after conversion takes the form of an analog voltage in accordance with the digital bits of the input digital signal.
In the described liquid crystal driving circuit, while lower four bits of the input digital signal are subjected to linear DA conversion, the upper two bits thereof are used to select the reference voltage. Thus, due to the fact that a kinked voltage having different DA conversion characteristics per 2.sup.4 =16 tones can be outputted by arbitrarily setting an intermediate voltage of each input reference voltage, it is possible to output an analog voltage which has been subjected to .gamma. correction, in accordance with the optical characteristic of the liquid crystal material.
In recent years, faced with a strong demand for finer liquid crystal panels capable of displaying more multi-tones, the competition for cutting down the cost of liquid crystal modules including the liquid crystal panel has not been higher. As a result, there also has been a strong demand for reducing the cost of a driving circuit for driving the liquid crystal panel, which is one of the peripheral devices other than the liquid crystal panel.
However, with the conventional driving circuit such as the one shown in FIG. 15, as the number of tones of a multi-tone liquid crystal panel is increased, the total capacitance of the capacitor alley 24 and the number of capacitors are inevitably increased. This increases the size of the driving circuit geometrically, resulting in an abrupt increase in chip size when integrated and a large increase in cost.
For example, in the circuit of FIG. 15, the total capacitance required for a single output circuit becomes 32 times that of the basic capacitance (2.sup.0 +2.sup.0 +2.sup.1 +2.sup.2 +2.sup.3 +2.sup.4 =1+1+2+4+8+16=32). Also, in this circuit, it is important to consider the relative accuracy between capacitance values to make sure the accuracy of DA conversion, and in order to realize accurate weighting ratio, it is required to provide a basic capacitance exceeding a certain value, taking into consideration the product dispersion.
Therefore, referring back to the 6-bit circuit of FIG. 15 as an example, in order to increase the number of bits of the digital signal to realize further multi-tones, it is required to abruptly set large values to the total capacitance of the capacitor alley 24 and to the capacitance of the feedback capacitor 27, resulting in an increase in chip size of the LSI and an increase in cost.
Also, for example, in the above conventional art, in the case of using the lower four bits of a digital signal carrying information of multi-tone display for DA conversion, it is required to provide four capacitors in the capacitor alley 24. That is, it is required to provide m capacitors for lower m bits of a digital signal (m is an integer of not less than 1), and for this reason, as the number of tones is increased in a multi-tone liquid crystal panel, the number of capacitors is also inevitably increased, resulting in an abrupt increase in chip size and a large increase in cost.